JavaScript RegExp g Modifier

The g (global) modifier in JavaScript regular expressions is used to perform a global search. It ensures the pattern is matched multiple times throughout the entire string, rather than stopping at the first match.

let regex = /cat/g; let str = "cat, caterpillar, catch a cat"; let matches = str.match(regex); console.log(matches);  

[ 'cat', 'cat', 'cat', 'cat' ] 

The g modifier finds all occurrences of "cat" in the string, even if they appear multiple times.

Syntax:

/pattern/g

Key Points

• Global Search: Finds all matches in the string, not just the first.
• Combination: Can be combined with other flags like i (case-insensitive) and m (multiline).
• Match Iteration: Useful for iterating over all matches using methods like match(), exec(), or loops.

Real-World Examples

1. Finding All Matches

let regex = /dog/g; let str = "dog, doggy, dogs are friends"; let matches = str.match(regex); console.log(matches);  

[ 'dog', 'dog', 'dog' ] 

Here, the g modifier ensures all matches of "dog" are found in the string.

2. Counting Word Occurrences

let str = "apple orange apple banana apple"; let regex = /apple/g; let count = (str.match(regex) || []).length; console.log(count);  

3 

Using the g modifier with match(), we can count how many times "apple" appears.

3. Replacing All Matches

let str = "foo bar foo baz foo"; let regex = /foo/g; let result = str.replace(regex, "qux"); console.log(result);  

qux bar qux baz qux 

The g modifier ensures all occurrences of "foo" are replaced with "qux".

4. Iterating Over Matches

let regex = /\d+/g; let str = "The price is 20 dollars and 30 cents."; let match; while ((match = regex.exec(str)) !== null) {  console.log(`Found: ${match[0]} at index ${match.index}`); } 

Found: 20 at index 13 Found: 30 at index 28 

The g modifier with exec() allows iterating over all numeric matches in the string.

5. Case-Insensitive Global Search

let regex = /hello/gi; let str = "Hello, HELLO, hello"; let matches = str.match(regex); console.log(matches);  

[ 'Hello', 'HELLO', 'hello' ] 

Combining g with i makes the search case-insensitive while finding all matches.

When Not to Use the g Modifier

If you only need the first match, do not use g. For example, str.match() without g returns only the first match as an array.

let str = "repeat repeat repeat"; console.log(str.match(/repeat/));  

[ 'repeat', index: 0, input: 'repeat repeat repeat', groups: undefined ] 

When using test() in loops, the g modifier can cause unexpected results due to its effect on the lastIndex property.

Why Use the g Modifier?

• Comprehensive Matching: Ensures no match is missed in the target string.
• Text Analysis: Useful for tasks like word frequency analysis or log parsing.
• String Manipulation: Ideal for global replacements and transformations.

Conclusion

The g modifier is essential for working with patterns that occur multiple times in a string, making it a powerful tool for developers in text processing and manipulation.

Recommended Links:

• JavaScript RegExp Complete Reference
• JavaScript Cheat Sheet-A Basic guide to JavaScript
• JavaScript Tutorial